SparkFun Forums

[Adenosine]

I'm planning to use something which will send 3.3v high/low signals to an Arduino, and I was wondering if connecting the 3.3v to the gate of a transistor (with both devices on a common ground) would allow 5v out to be read on an Arduino as high?

[MichaelN]

An Arduino running on 5V will accept a 3.3V signal as a valid logic "high" signal, so you don't actually need any interface circuitry (have a look at the datasheet for the microcontroller used).

If you really want to use a transistor, the following circuit will do what you want, and will also provide some protection for the Arduino. Rpullup and Rbase are normally between 10K and 100K; the values aren't critical.

[n1ist]

That circuit will work, but it's an inverter. Make sure you take that into account.
/mike

[Adenosine]

An Arduino running on 5V will accept a 3.3V signal as a valid logic "high" signal, so you don't actually need any interface circuitry (have a look at the datasheet for the microcontroller used).

If you really want to use a transistor, the following circuit will do what you want, and will also provide some protection for the Arduino. Rpullup and Rbase are normally between 10K and 100K; the values aren't critical.

Tested it out, and it works, but how can I hook it up so the signal is sent HIGH when it is pressed and LOW when not pressed? It currently works in the opposite.

[waltr]

Add a second transistor to re-invert the logic.

[Adenosine]

Add a second transistor to re-invert the logic.

Could you explain what that means? I'm still new to all these electronics terms and stuff.

[markaren1]

Before you get too stuck into solution, what is the problem ?

What are you trying to interface to/from ?

-mark

[Adenosine]

Before you get too stuck into solution, what is the problem ?

What are you trying to interface to/from ?

-mark

I paid about $5 for a remote controlled car and it outputs to 3.0v, and works at the 3.3v so I'm using that as a cheap remote controller system. I set up the diodes in a way where it will output to this wire if pressing forward and a different wire if reverse. Same concept for L/R so I have 4 buttons.

[waltr]

Add a second transistor to re-invert the logic.

Could you explain what that means? I'm still new to all these electronics terms and stuff.

In the schematic above +3V on the input rises the transistor's base which turns on the transistor and the collector goes to ground as the output. Connect this output through a resistor to the base of another transistor. Now the second transistors base is at ground, the second transistor is off and its collector is pulled to +5 volts with a resistor.

Double inverting of your input signal from +3V to ground to +5V.

[markaren1]

I paid about $5 for a remote controlled car and it outputs to 3.0v, and works at the 3.3v so I'm using that as a cheap remote controller system. I set up the diodes in a way where it will output to this wire if pressing forward and a different wire if reverse. Same concept for L/R so I have 4 buttons.

A 3V signal may be able to control 5V logic directly. You usually need 70% of VDD (0.7 * 5.0 = 3.5V), but 3V may be enough.

Might be worth putting 10k between the remote control and the uP - just in case...

-Mark

[markaren1]

...and if my suggestion doesn't work, go with MichaelN's suggestion - it just means that the polarity of the signals will be inverted when you read them which is easy to fix in software...

[MichaelN]

I just had a look at a few ATMEGA datasheets, and the minimum valid input voltage for a logic "1" is 0.6*Vcc ie 3V when running on 5V.

As mentioned, it may be a good idea to use a transistor interface anyway, as this will give some protection for the Arduino against spikes etc.

It might be worth adding a pulldown resistor too; otherwise the transistor will start to switch on with voltages above about 0.6V. The ratio of Rbase to Rpulldown controls the threshold voltage at which the transistor will switch on.

[macegr]

No need to do all that.

[MichaelN]

No need to do all that.

Interesting idea, but when the input line is pulled low, the output voltage will still be about 2.4V, which is not low enough for a valid logic "0" on an ATMEGA (the datasheet specifies a maximum of 0.3*Vcc, ie 1.5V).

[markaren1]

Input on a Collector ?